To remap the seach button to something instead of search, put something along the lines of the following into ~/.xbindkeysrc
Example here is to remap it to alt+f4 to close a window.

"/usr/bin/xvkbd -text "\[Alt_L]\[F4]""
c:0xE1

Alternatively, Gnome will recognize the small middle search button as a keyboard event. Thus, you just have to go into keyboard shortcuts and remap that to something. This is probably the most reliable way to go about using this key.